Nuclear Magnetic Resonance (NMR) as a well logging technique does not always yield useful results. Part of the problem with NMR logging is a consequence of faulty assumptions, particularly as they apply to carbonate rocks. Carbonates are particularly troublesome because great variations in their pore sizes and organic materials distributed throughout the grains can cause misleading results. Conventional interpretation of NMR measurements is based on a number of potentially faulty assumptions, to wit:
1) In addition to the bulk relaxation mechanism, magnetization decays because water molecules diffuse to the surface of the grain where they experience an enhanced relaxation rate. This is assumed to be a result of their interaction with local magnetic fields associated with paramagnetic impurities in the grain.
2) The pore space is divided into separate pores that do not interact with each other.
3) Within each pore, magnetization is assumed to be uniform. The basis for the latter assumption is that .rho.V.sub.s /D&lt;&lt;1, where D is the diffusion coefficient in the bulk fluid, V.sub.s is the volume to surface ratio of the pore, and .rho. is the surface relaxivity. The characteristic decay time constant of spin-spin relaxation of any pore is then given as: EQU 1/T.sub.2 =.rho./V.sub.s +1/T.sub.2b (1)
where T.sub.2b is the bulk relaxation time.
4) The magnetization decay can be represented as an integral of contributions of all such components due to the multitude of pores of different volume to surface ratio. If the probability density function of T.sub.2 is g(T.sub.2), this integral can then be resolved into the components represented by g(T.sub.2) by a number of established fitting procedures. In practice, g(T.sub.2) is widely considered to represent the pore size distribution.
The above assumptions are flawed in general, and particularly several of them do not hold for carbonate rocks. The measured decay is not readily translatable to pore sizes. (Assumption 4). Inverted T.sub.2 distributions in grain-supported carbonates are unimodal, whereas petrography studies show them to be at least bimodal. Petrographs show that the grains are composed of micrite particles, which form the intragranular porosity. The juxtapositioning of pores of diverse sizes, and the diffusion of magnetic moments among these pores, causes the breakdown of the relationship between T.sub.2 and pore size. (Assumption 3).
Most recently, we have discovered that NMR tests performed on carbonate rock samples from Middle East oil wells are temperature dependent. The temperature dependencies are contrary to established beliefs for NMR response in fluid-saturated rocks. The prior art was based on the supposition that there were no significant temperature dependencies for the NMR response. Closer examination of this work suggests that these conclusions were valid only for sandstone media. Data obtained for carbonate media were simply inconsistent. Furthermore, a recent comparison of laboratory core data with data obtained from logs taken in the Middle East wells also shows inconsistencies.
The studies made in accordance with the present invention confirm the prior art conclusion with respect to a limited number of sandstone media. That is, the change in response to temperature is weak or non-existent. However, core samples taken from the Middle East wells cited in the comparative data show a dependence on temperature which is consistent with the observed discrepancies between laboratory core data and log data.
The extent of the temperature dependence is very significant for NMR logging interpretation. In particular, the traditional practice of using an empirically determined "cut-off" in the relaxation time distribution becomes completely invalid unless extensive corrections are made to correlations obtained at room temperature.
In the context of more advanced models of NMR in water-saturated rocks, the data is explained only by a temperature dependence of the intrinsic nuclear magnetic relaxivity of the rock pore surfaces. The interpretation methodology recently proposed for dual porosity carbonate systems of the type known as "peloidal grainstones" or "peloidal packstones" still delivers an expected length scale for large, intergranular pores, but fails to deliver a correct length scale for micropores unless it is extended using a laboratory determination of relaxivity as a function of temperature.
Selecting a laboratory correlation appropriate for the known geological and mineralogical characteristics of the sub-surface known a priori from other data, or determined in situ using other logging tools, it is possible to adapt the interpretation of the carbonate rock to account for the temperature dependence. Thus adapted, the methodology delivers length scales for both the intergranular pores and the micropores of the dual porosity systems common in carbonate geology. Although the inferred intergranular pore length scale should be affected only weakly by the temperature effect disclosed herein, the micropore length scale will be affected strongly. Both of these length scales are important in estimating transport properties such as electrical conductivity and hydraulic permeability. The latter is of considerable importance in evaluating subsurface formations, because it determines how easy it is to produce the hydrocarbons which may be present. The former is important, because it affects the interpretation of associated resistivity logs. The resistivity logs are a primary source of data for estimating how much hydrocarbons may be present.
The present invention contemplates two equivalent methods for obtaining reliable NMR results for bound fluid and free fluid in carbonate rock formations based upon adjusting T.sub.2 data with respect to temperature. The invention also features a more advanced methodology for ascertaining the rock petrophysical properties, which methodology adapts another application to include the newly discovered temperature dependence of relaxivity.